1. Field of the Invention
The present invention relates to a method of controlling fuel injection in an internal combustion engine which employs a single point injection system for supplying fuel to a plurality of cylinders by a single fuel injection operation of a plurality of fuel injectors or fuel injection valves disposed on an upstream side of a collector portion of an intake manifold of the internal combustion engine.
2. Description of the Related Art
There have been conventionally a single point injection (SPI) system in which a fuel injection valve is disposed on an upstream side of a collector portion of an intake manifold of an internal combustion engine so as to supply fuel to each of cylinders and a multi point injection (MPI) system in which one fuel injection valve is provided in each of the cylinders so as to supply fuel to a respective one of the cylinders.
The SPI system is employed in some internal combustion engines at present because the SPI system has a simpler structure than that of the MPI system, and the SPI system is frequently used, for example, in an engine of a fork lift truck or the like.
In general, a fuel injection apparatus of an internal combustion engine employing the SPI system is provided therein with a single fuel injection valve so as to carry out fuel injection in correspondence to an operating condition of the engine by controlling a fuel injection time.
In the case where the injection time is varied in correspondence to the operating condition on the basis of the injection start timing of the fuel injection valve, if the fuel injection time is increased or reduced due to dispersion of an engine control unit or the like and the fuel injection start timing is unchanged under the same operating condition, a distribution ratio of the fuel to be supplied to each of the cylinders is disequilibrated. In other words, some cylinders having a lean fuel mixture and others having a rich fuel mixture come into existence, resulting in some disadvantages.
Accordingly, there has been proposed a method of controlling an injection operation by calculating an intermediate time between a fuel injection start and a fuel injection end and determining the fuel injection start timing on the basis of the intermediate time of the fuel injection. Such a method is disclosed in Japanese Patent Application Laid-Open Publication No. 10-196441.
In general, the SPI system employs a single injector for controlling fuel injection. In the case where the SPI system is used in a large displacement engine or the like, the injector is required to have a large capacity. However, there is a problem that the large capacity causes a reduction in the ability to precisely set the fuel injection amount.
Further, although it may not be regarded as a complete SPI system, Japanese Patent Application-Laid-Open Publication No. 5-87021 discloses a control method which is constructed in such a manner that a fuel injection valve is provided in each of cylinders and an additional fuel injection valve is disposed near a collector portion of an intake manifold, whereby fuel is injected and supplied only from the fuel injection valve provided in each of the cylinders in the case of a low load, and the fuel is injected and supplied from both the fuel injection valve provided in each of the cylinders and the additional fuel injection valve provided near the collector portion of the intake manifold in the case of a high load.
It is also known to control fuel injection in the SPI system in correspondence to a fluctuation of the load by using two fuel injection valves, as shown in FIGS. 5A and 5B by way of example.
This technique, which is used in, for example, a six-cylinder engine, is constructed so as to control a fuel injection amount on the basis of measured intake airflow information from an air flow meter incorporated in a fuel injection control apparatus. In this technique, only one of the two fuel injection valves injects fuel so as to sequentially supply the fuel to each of the cylinders in the case of a low flow rate, and the two fuel injection valves are simultaneously operated so as to sequentially supply the fuel to each of the cylinders in the case of a high flow rate. In FIGS. 5A and 5B. for example, reference symbols #1CYL to #6CYL denote the first cylinder to the sixth cylinders, respectively, and reference symbols #1INJ and #2INJ denote the first and second fuel injection valves or injectors, respectively.
Further, in addition to the technique mentioned above, there is a method of simultaneously operating two fuel injection valves every time regardless of the intake airflow rate.
However, in the case of the above mentioned technique in which the control is executed in correspondence to the fluctuation of the load by using two fuel injection valves in the SPI system, if only one fuel injection valve is operated in the case of the low flow rate, said only one fuel injection valve is used very often, so that durability thereof is deteriorated. Further, if the two fuel injection valves are always operated at the same time, the ability to precisely set the fuel amount in the case of the low flow rate becomes low and thus the precision of the fuel injection is lowered, so that the fuel distribution to each of the cylinders is deteriorated.